After a well has been completed to produce oil or gas, it is necessary to periodically service the well. There are many occasions where the service procedure is carried out using coiled tubing. Such tubing is inserted into the wellhead through a lubricator assembly or stuffing box. Typically, this is necessary because there is a pressure differential at the surface of the well and the atmosphere, which may have been naturally or artificially created, that serves to produce oil or gas or a mixture thereof from the pressurized well. The tubing that is inserted into the well is normally inserted through a lubricator mechanism which provides a seal about the O.D. of the tubing for the retention of any pressure that may be present at or near the surface of the well. The tubing is inserted by a coiled tubing injector apparatus which generally incorporates a multitude of gripper blocks for handling the tubing as it passes through the injector. The tubing is flexible and can therefore be cyclically coiled onto and off of a spool, or reel, by the injector which often acts in concert with a windlass and a power supply which drives the spool, or reel.
The coiled tubing injector apparatus utilizes a pair of opposed endless drive chains which are arranged in a common plane. These opposed endless drive chains are often referred to as gripper chains because each chain has a multitude of gripper blocks attached therealong. The gripper chains are driven by respective drive sprockets which are in turn powered by a reversible hydraulic motor. Each gripper chain is also provided with a respective idler sprocket to maintain each gripper chain within the common plane. Both the drive sprockets and idler sprockets are mounted on a common frame wherein the distance between centers of all the sprockets are essentially of a constant distance from each other. That is, the drive sprockets are free to rotate but are not free to move either vertically or laterally with respect to each other. The idler sprockets are not free to move laterally with respect to each other, but are vertically adjustable within a limited amount in order to set the amount of play in each gripper chain. Such vertical adjustment is done by either a mechanical adjusting means or a hydraulic adjusting means. Typically, for injectors having mechanical adjustment means, the adjustment is made when the injector is not in operation.
The opposed gripper chains, preferably via the gripper blocks, sequentially grasp the coiled tubing that is positioned between the opposed gripper chains. When the gripper chains are in motion, each chain has a gripper block that is coming into contact with the coiled tubing as another gripper block on the same gripper chain is breaking contact with the coiled tubing. This continues in an endless fashion as the gripper chains are driven to force the tubing into or out of the wellbore, depending on the direction in which the drive sprockets are rotated. Preferably, gripper blocks such as those set forth in U.S. Pat. No. 5,094,340, issued Mar. 10, 1992, to Avakov, which is incorporated herein, are used.
Because the gripper chain drive sprockets and idler sprockets are essentially in a fixed relationship with each other, the gripper chain is provided with a predetermined amount of slack which allows the gripper chain to be biased against the coiled tubing to inject the tubing into and out of the wellbore. This biasing is accomplished with an endless roller chain disposed inside each gripper chain. Each roller chain engages sprockets rotatably mounted on a respective linear bearing beam. A linkage and hydraulic cylinder mechanism allows the linear bearing beams to be moved toward one another so that each roller chain is moved against its corresponding gripper chain such that the tubing facing portion of gripper chain is moved toward the tubing so that the gripper blocks can engage the tubing and move it through the apparatus. The gripper blocks will engage the tubing along a working length of the linear beam.
Each chain has a gripper block that contacts the tubing at the top of the working length as a gripper block on the same chain is breaking contact at a bottom of the working length of the linear beam.
The fixed distance between each set of gripper chain drives and idler sprockets requires some significant lateral movement in the gripper chain when engaged by the roller chain on the corresponding linear beam in order to allow the gripper chains to engage the tubing by way of the gripper blocks. The reason for having the requisite amount of lateral play in the gripper chains is to provide a limited amount of clearance between the gripper chains, upon moving the respective roller chains away from the vertical center line of the injector, to allow the passage of tubing and tools having larger outside diameters or dimensions. An inherent shortcoming in this design is that the required slack can often cause misalignment problems and even binding problems with the chains due to having to accommodate ever increasing outside nominal dimensions of downhole tools and wellhead equipment. Another troublesome characteristic manifests itself in the large approach and departure angles defined by the region where the respective paths of the gripper chains converge upon, and diverge away from, the working center line of the injector wherein the coiled tubing is preferably positioned for being injected or extracted into or out of the well. The large approach and departure angles reduce the overall efficiency of the injector due to the necessity of supporting and contending with the reactive forces generated by the chains when the injector is in operation.
A further, if not more predominate reason why large gripper chain approach and departure angles are not desired, is that large angles tend to increase the likelihood of the chain-mounted gripper blocks to mark, or gouge, the tubing as the blocks come into contact with the tubing. Such marks, or gouges, create stress risers within the wall of the tubing which can lead to premature structural failure of the tubing. A tubing injector apparatus not having large gripper chain approach or departure angles, yet being able to accommodate large diameter tubing and wellhead equipment would advance the art considerably.
Therefore, there is a need within the art to provide an injector that, while the injector remains installed about the wellhead, can accommodate large nominal diameter tools and surface equipment, yet can provide efficient and reliable chain operation to generate the high forces needed for injecting and extracting tubing of long lengths and large diameters into and out of the wellbore.
Another need within the art is for an injector having the ability to accommodate a wide range of tubing diameters while in operation. Such an injector would allow for improving operations wherein coiled tubing having differing diameters that have been connected to each other to form a single string of tubing are being used in the servicing of the well.
Another need within the art is for an injector that can accommodate the ever-increasing nominal outside diameters of tubing while avoiding: chain misalignment, chain binding tendencies, improper chain tension, gripper block marking or gouging, and other inherent design problems of prior injectors which manifest themselves when working with tubing, tools, and surface equipment having large nominal outside diameters.
One coiled tubing injector apparatus which resolves the foregoing problems is the apparatus described in patent application Ser. No. 08/508,411 entitled TWIN CARRIAGE TUBING INJECTOR APPARATUS, assigned to the assignee of the present invention, the details of which are incorporated herein by reference. However, the apparatus described therein, along with prior injectors, do not address other difficulties associated with injecting, suspending and extracting coiled tubing from a wellbore.
Generally, as provided herein, the wellbore in which the tubing is injected will be pressurized, so that as the tubing is initially inserted through the injector and into the wellbore, the pressure will tend to resist injection of the tubing. In other words, when the length of tubing in the well is such that it is insufficient to overcome the pressure in the wellbore, the pressure will tend to resist injection and will act to force the tubing upward. At some point, the weight of the tubing will overcome the pressure in the wellbore. The weight of the tubing will then apply a downward vertical load to the gripper blocks that engage the tubing. The downward load is typically called a "hoisting" load.
When the gripper chains are in motion, the gripper blocks along the working length of the linear beam engage the tubing. The lateral load applied to the tubing by the opposed gripper chains will generally be uniform along the working length of the linear beam. The vertical hoisting load is carried by linking pins which connect the gripper blocks together to form the endless gripper chains. Because the lateral load applied to the tubing is substantially uniform, the vertical load applied to the gripper chain by the tubing will be carried primarily by the linking pins which connect the gripper blocks at the lower or bottom end of the working length of the linear beam. The linking pins connecting the remainder of the blocks carry little or no vertical load. Because the vertical load during hoisting is concentrated on the lower linking pins, the life of the chain is reduced.
Thus, there is a need for a coiled tubing injector apparatus which will accommodate large nominal diameter tools and tools of differing diameters while in operation, and at the same time redistribute the vertical load created by the tubing from the linking pins at the bottom of the working length to all of the pins along the working length of the linear beam and the corresponding working length of the gripper chain.